1. Field of the Invention
The present invention relates to lead-free glass tubing or pipes, especially for use in a method of encapsulating diodes and also to diodes made thereby.
2. Description of the Related Arts
Diodes are semiconductor electronic components, which are characterized by a strongly asymmetric current-voltage characteristic curve. Diodes are predominantly used for rectification of alternating current.
Conventional diodes, especially low-power diodes, comprise a silicon chip, which is pre-passivated along its periphery and which has a contact between an n-type semiconductor and a p-type semiconductor, terminals made of cooper-clad wire and/or copper head pins and a glass tube, which encapsulates the diode including the terminal positions. One type of terminal comprises a core made of, for example Ni/Fe-42-alloy or Ni/Fe-47 alloy, which has a copper surrounding jacket, which is coated with Cu2O. The contacting between the chip and the terminals occurs by pressure, which is maintained through the glass tubing.
Glasses for hermetic encapsulation of diodes, short diode glasses, must be adjusted in their thermal expansion properties and their viscosity temperature dependence to the alloy used for the metal conductors, so that peeling off or chipping off of glass from the terminal wires does not occur either in the manufacture of the diodes or during temperature variations. The Cu2O coating acts as an adhesive between the glass and the copper cladding. For a protective encapsulation the viscosity values of the glass should be such that processing can occur at low temperatures and short melting times. Typical numbers here are about 10 minutes at about 630° C. The diode glass should thus have a processing temperature (temperature at which the viscosity is 104 dPas)≦850° C., a softening temperature EW≦600° C. and a transformation temperature Tg of ≦480° C. Since the glass tubing section forms the diode body after melting, whose dimensions are predetermined and which should be satisfactorily printable, it may not be shaped substantially on melting in spite of good glassfication. The melting is thus performed at temperatures at which the glass has a viscosity of about 106 dPas,
In order to guarantee sufficient mechanical workability of the diodes, the diode glass should have an elastic modulus of >50 Gpa. If the diodes are encapsulated with a glass with a lower elastic modulus, they have an increasing tendency to peel or chip off from the board or plate, on which they are soldered.
Diode glass is produced as tubing. Typical tube dimensions are the following: outer diameters between 1.1 mm and 4.5 mm; salable dimensions are currently especially outer diameters of 1.4 mm, 1.52 mm and 1.78 mm. Typical interior diameters are between 0.65 mm and 0.86 mm. Besides the typical frequent round form of the diodes, diodes with square outer contours are known. These diodes and also diodes with other shapes are made in known ways.
For production of glass tubing, also especially with the named small diameters, high devitrification stability is required, so that crystal formation does not occur during tube drawing, e.g. from an outlet orifice or nozzle. The upper devitrification temperature, OEG, (liquidus temperature), is a measure of the devitrification stability. In the present case it should be at least 50 K under the processing temperature VA. Moreover the growth speed of the crystals should be as small as possible. Generally there is a temperature Kgmax (<OEG) at which the growth speed of the crystal is greatest, vmax. This vmax should also be as small as possible.
The glass furthermore should have a sufficient resistance to washing processes in tubing manufacture and diode process. This is guaranteed with a class three water resistance according to DIN ISO 719.
A essential characteristic of a diode is its current-voltage characteristic in the blocking or high-resistance direction. The behavior or current-voltage curve should show a drastic or dramatic increase without notable edge rounding when the blocking or high-resistance voltage is exceeded. Also the blocking voltage of all diodes of one type should have the same value. The encapsulated glass may thus not impair the effectiveness of the diode, i. e. it may not contain damaged parts.
These requirements are fulfilled by the commercially obtainable diodes and diodes glasses. These diode glasses are alkali-poor, and are practically sodium-free and lithium-free although they contain up to the level of the usual impurities of these ingredients. However they can contain up to five percent by weight K2O and they contain an arbitrarily high concentration of PbO up to 60 percent by weight.
However PbO is environmentally unfriendly and legal considerations prevent the use of this ingredient in electrical and electronic devices. This has led to the goal of producing PbO-free glasses, which are suitable for encapsulating diodes and to lead-free diodes. An economical reproduction of the desired glass engineering properties influenced by PbO by replacing lead oxide with one or more other sufficiently available and economical ingredients has not succeeded up to now.
Also other electronic units, such as precision resistors, ceramic capacitors, tantalic capacitors or LEDs may be encapsulated like the diodes. The housings of these components have the same requirements as described for the diode glasses above.
EP 1 156 020 A1 describes PbO-free diode glasses, which contain B2O3 as a required ingredient and at least two alkali metal oxides, selected from the group consisting of Li2O, Na2O and K2O. In practice the disadvantage of these glasses is a reduced water resistance and a high mechanical susceptibility to failure.